This invention relates to lasers and more particularly to organic dye lasers and to the use of certain nitrogen-containing dyes capable of lasing when properly excited.
Apart from their previous use as laundry brighteners, coumarin derivatives 161.5.degree.; been the subject of a considerable amount of research because of their usefulness as lasing dyes. Snavely, for instance reports in U.S. Pat. No. 3,521,187 that, generally, any fluorescent coumarin dye which is soluble in water may be used as a laser dye. Drexhage in U.S. Pat. No. 3,873,940 discloses a vast array of coumarin derivatives with a fused rigidized nitrogen containing ring which can be useful as laser dyes. Drexhage emphasizes the necessity that the nitrogen be part of a heterocyclic ring which is joined to the main ring at at least two points. Drexhage and Reynolds reported in Optics Communications Vol. 13, p. 222 (March 1975) that they and others have found that coumarin derivatives with an amino group in the 7 position are very efficient laser dyes in the blue and green regions of the spectrum. The inventors also have done previous work with 7-amino coumarin, as reported in Optics Communications, Vol. 11, p. 352 (1974).
The numbered positions of the coumarin ring structure are as follows: ##STR1##
7-Amino coumarin laser dyes previously described have had other substituents on the ring structure, typically in the 4 position.
4-Methyl-7-diethyl-amino coumarin was attractive for use as a laser dye because it is commercially available and it lases in the blue-green range of the spectrum, which is important in underwater laser systems. This and similar compounds, however, have unsatisfactory photochemical stability. 4-Trifluoromethyl coumarins were found to be more stable, but they lase 40 to 50nm more to the red than the unfluorinated dye.
7-Amino coumarins are thought to photodimerise across positions 3 and 4, and workers in the field have previously added electron accepting or bulky substituents at these positions to increase the dyes' stability.